Polarization is the phenomenon, which is exhibited when a transverse wave is polarized. Light is a transverse electromagnetic wave that vibrates in a direction perpendicular to the direction of propagation. Ordinary light is unpolarized, consisting of a mixture of waves vibrating in all directions. In an unpolarized wave, the vibrations in a plane perpendicular to the ray appear to be oriented in all directions with equal probability. Light is said to be polarized if the direction of the vibrations is completely predicable and remains constant for some reason.
There are different ways for producing linearly polarized light, for example by using so called polarizers. A polarizer is a device from which the emergent beam is linearly polarized. The direction of vibration of the emergent linearly polarized light is a property of the polarizer and is referred as the polarizer's axis. Since most polarizers operate in transmission, a frequently met term is transmission axis. A polarizer is transparent only to incident light polarized in the direction of the polarizer's axis. Thus, the combination of two polarizers is opaque when the polarizers are oriented with their axes perpendicular to each other. If the second polarizer is rotated with the axis no longer perpendicular to that of the first one, the amount of light transmitted gradually increases until the two polarizers are parallel to each other.
The plane of a polarized light can also be rotated by a fixed angle when it passes through a medium or a device called optical rotator or Faraday rotator. The direction and the amount of the rotation (rotation angle) depend on the initial properties and thickness of the medium or device.
A retarder or waveplate is a device that resolves a light wave into two orthogonal linear polarization components along two particular orthogonal directions fixed in the retarder and produces a phase shift between them. The phase shift is a property of the retarder, referred as the retarder's retardation (Δ) and the particular direction with respect to which retardation is produced is known as the reference axis of the retarder. The angle of the reference axis of a retarder in a system containing it is referred as the retarder's orientation angle (φ). The most common retarders introduce phase shift or retardation of 90° and 180° and they are called quarter-wave and half-wave retarders. A normal birefringent retarder is a plane-parallel plate of birefringent material whose optic axis is parallel to faces of the plate.
Polarization interference filters or birefringent filters are based on the interference of polarized light. Such filters consist of birefringent plates and polarizers. The retardation Δ=Δ(λ) of a birefringent plate is dispersive, i.e. depending on the wavelength λ of light incident on it. The dispersion of the parameter retardation Δ(λ) forms the basis of a birefringent filter with the transmitted light intensity varying as a function of the wavelength λ. The initial birefringent filters are not tunable, having their wavelength selections of light fixed.
Their tuning methods are introduced below and further in more detail in the section FURTHER BACKGROUND.
The spectral transmission, or simply transmission, of a (spectral) filter is a property of the filter, defined as the light intensity transmitted by the filter varying as a function of the wavelength of incident light relative to the light intensity incident on the filter. The latter can be considered constant in most cases so that the spectral transmission or transmission of a filter is equivalent to the intensity transmitted by the filter as a function of light wavelength.
The polarization interference filter or birefringent filter is a kind of active filter with the transmission of the filter under control of the designer. The filter can be manufactured to have very high resolution and system performance. Another advantage is its superior image quality so that it is known as the image-quality filter and especially suitable to be used in imaging equipment. Further major advantages include possibly high acceptance angle and large clear aperture. The birefringent filter has proven an effective tool in astronomical research and it has unique value especially in solar physics. Due to its superior image quality the filter has been widely used in imaging equipment, such as microscopes and imaging spectrometers. The filter has also been used as tuning devices in monochromator and tunable laser. Some other important applications include optical telecommunication and radar, imaging and projection, color pattern analysis and display, remote sensing and space-based devices.
The birefringent filter was first invented by Lyot (Lyot, B. (1933) Comptes Rendus 197:1593). The basic Lyot filter (Yariv, A. and Yeh, P. (1984) Optical Waves in Crystals, Chapter 5, John Wiley and Sons, New York) consists of a set of birefringent crystal plates sandwiched by parallel polarizers. The thickness of each birefringent plate is twice that of its preceding one and all the plates are oriented at an azimuth angle of 45°. Another type of birefringent filter is {hacek over (S)}olc filter ({hacek over (S)}olc (1965), J. Opt. Soc. Am. 55:621). A {hacek over (S)}olc type filter is an optical network comprising identical birefringent plates arranged in series between a pair of polarizers and oriented properly. A Lyot filter generally requires fewer birefringent plates than an equivalent {hacek over (S)}olc filter. However, the {hacek over (S)}olc filter uses no intermediate polarizer, therefore can provide higher transmission. The theory and art of the Lyot and {hacek over (S)}olc filters as well as their tuning methods have been reviewed and/or discussed by Evans (Evans, John W. (1949) J. Opt. Soc. Am. 39:229) and Title and Rosenberg (Title, A. M. and Rosenberg, W. J. (1981) Opt. Eng. 20:815). Also of interest are articles of Gunning and Lotspeich et al (Gunning, W. J. (1981) Opt. Eng. 20:837; Lotspeich, J. F., Stephens, R. R. and Henderson, D. M. (1981) Opt. Eng. 20:830).
Birefringent plates used in a Lyot or {hacek over (S)}olc type filter are parallel plates or sheets made by crystal materials such as quartz and calcite cut such that the optic axis is parallel to the plate surface, or by other birefringent materials. The crystal materials are suitable for applications requiring high resolution and/or low wave-front distortion. The other birefringent materials include, but not limited to, polymer materials, such as polyvinyl alcohol and polycarbonate, and liquid crystal polymer films. These materials are attractive, permitting filters of large clear aperture and/or low costs, with this enabling many potential applications of the filter. The initial Lyot or {hacek over (S)}olc filter is not tunable. The usefulness of an optical spectral filter will be greatly increased if its wavelength-selection is tunable. Tuning a birefringent filter is to simultaneously change or shift the retardation of all the constituent birefringent plates of this filter. Conventionally there are three basic technologies for tuning birefringent filters, i.e. direct method, rotating-element method and electro-optic tuning. The direct method is to change the physical thickness of the tuned birefringent plate, for example, by using wedge-shaped retardation plates in pairs. Although it is simple, but it is not the most practical tuning method because a change in the optical path is undesirable for imaging systems and temperature control is needed.
The rotating-element method is to use achromatic retarders, usually birefringent achromatic waveplates. This is a very effective tuning method used in practice. A tunable birefringent filter using this method can be manufactured to reach very high resolution and system performance. According to the art (see below), generally three achromatic waveplates are required for tuning a single birefringent plate. In special cases, the number of the achromatic waveplates required can be reduced, but at least one achromatic quarter-wave plate is necessary. An achromatic waveplate usually used for this purpose is a compound component manufactured by combining two or more single birefringent plates. To tune a single element plate, thus it is normal to require several birefringent plates in average or even more so that an assembled tunable filter becomes a fairly complex optical-mechanical assembly although its initial wavelength-fixed structure could comprise only a few plates. Also for this reason, the manufacture costs of a tunable filter made by birefringent crystal material based on this method are very high and further enhancement of the resolution, system performance or transmission is seriously restricted. On the other hand, it is the high expense and complex structure of a tunable birefringent filter that prevents its many potential applications. Another liability for a filter using achromatic waveplate(s) is the restriction of the achromatic waveplate(s) on the spectral range of the filter. For such filter, usually it is the operating spectral range of the achromatic waveplate(s) that determines and seriously restricts the wavelength range of the filter. Broadband achromatic waveplates are very difficult and expensive to be made.
For the electro-optic tuning there is no moving part and the speed of tuning can be very high. Optical spectral filters that can be rapidly tuned are attractive especially in applications of signal processing, color display, space-based platforms, remote sensing and wavelength division multiplexing. Tunable birefringent filters can be constructed by using electro-optic crystal plates or modulators (see e.g., Gunning, W. J. (1981) Opt. Eng. 20:837; Lotspeich, J. F., Stephens, R. R. and Henderson, D. M. (1981) Opt. Eng. 20:830). There are many versions of the filter design and they are developed based on the basic Lyot or {hacek over (S)}olc filter structures or in analog with the mechanical tuning methods. Tuning can be accomplished by electrically changing the birefringence of electro-optic birefringent plates or replacing a rotating achromatic waveplate with its electro-optic equivalent. The main disadvantage of the electro-optic tuning is small clear aperture and field-of-view. Another limitation is that the transmission of a filter of several stages can significantly be reduced by the electrodes for a longitudinal electric field or the voltages required for an aperture up to several centimeters becomes extremely high alternatively if a transverse electric field is used. In addition, the electro-optic tuning is very expensive.
As an alternative to electro-optic materials, liquid crystal (LC) cells or switches have been used for constructing switchable and tunable filters based on the Lyot or {hacek over (S)}olc structures or the basic configurations of the mechanical tuning methods (see, e.g. Tarry, H. A. (1975) Elect. Lett. 18:47; Scheffer et al, U.S. Pat. No. 4,019,808; Kaye, W. I., U.S. Pat. No. 4,394,069; Johnson et al, U.S. Pat. Nos. 5,132,826 and 5,231,521; Miller, P. U.S. Pat. No. 5,689,317; Sharp et al, U.S. Pat. No. 6,091,462). There are various LC devices (refer e.g. Saleh, B. E. A. and Teich, M. C. “Fundamentals of photonics”, John Wiley & Sons Inc, 1991; Clark, N. A. et al. (1983) Mol. Cryst. and Liq. Cryst. 94:213; and Anderson et al (1987) Appl. Phys. Left. 51:640). They include, for example, nematic and homeotropically aligned nematic LC cells, ferroelectric LC (FLC) cells, surface-stabilized ferroelectric LC (SSFLC) cells, smectic A* (SmA*) LC cells and distorted helix ferroelectric LC (DHF) cells, and twisted-nematic polarization rotators. While having drawbacks such as viewing angle and contrast ratio, the liquid crystal device has a few attractive features including compact size, low cost, large clear aperture and low power requirement so that it is particularly useful not only in display technology but also in many other applications.
As the prior art demonstrates, the existing methods for constructing tunable birefringent spectral filters require expensive and complex structure and/or they are restricted for their liabilities.